“Point” measures of HPA activity (plasma or salivary cortisol) are often used as biomarkers of temperament or other psychological variables, but these measures only reflect HPA activity over minutes. Hair cortisol reflects HPA activity over months, but has never been studied as a predictor of primate social behavior. To test the hypothesis that hair cortisol is a better predictor of social behavior than salivary cortisol, we studied two cohorts of differently reared rhesus infants (born 1 year apart) during the first 6 months of life (N=48; n=24 in each cohort; n=16 in each rearing condition: mother-peer-reared, peer-reared, and surrogate-peer-reared). We observed exploratory, self-directed, withdrawal, play, and other social behaviors over time and then measured hair cortisol at month 6. Because the cohorts differed significantly in their hair cortisol values [ANOVA; F(1,46)=11.85; p<0.01] each cohort was tested separately. Linear regression revealed that rearing condition was a significant predictor of withdrawal, social play, and other social behaviors for both cohorts [multiple R>0.42;n=48; p<0.05]. Hair cortisol added significant predictive power to the regression model for self-directed behaviors for both cohorts [DR2>0.19; p<0.05], and for social play behavior for one cohort [DR2=0.04; p<0.01]. Baseline salivary cortisol did not add significant predictive power to the model for any of the behaviors [DR2<0.04; p>0.10]. These findings indicate that under the present conditions, long-term measures of HPA activity are superior predictors of infant social behavior.